1. Field of the Invention
The present invention relates to a light beam scanning apparatus for use in an image forming apparatus, such as a laser beam printer and the like.
2. Related Background Art
The composition of a conventional light beam scanning apparatus in, for example, a laser beam printer, is as shown in FIG. 1. Referring to FIG. 1, a laser beam L radiated from a laser source 91, such as a semiconductor laser element, is collimated by a collimator lens 92 and then radiated onto a deflecting reflection plane of a rotary polygon mirror 93. The polygon mirror 93 has a plurality of (for example, six) deflecting reflection planes and is rotated at constant speed by a constant speed rotary motor 94 in the direction of the arrow "a" in FIG. 1, thereby sequentially deflecting and scanning the light beam L incident thereon through the collimator lens 92 in the direction of the arrow "b" in FIG. 1. A scanning lens 95 is disposed in the optical path between the rotary polygon mirror 93 and a plane to be scanned, images the laser beam deflected and scanned by the rotary polygon mirror 93 in a spot onto a photoconductive drum 96 as a plane to be scanned, and scans the laser beam at a constant speed.
A light sensing element 97 for generating a horizontal synchronizing signal to determine an image start position on the plane to be scanned is mounted on the scan start side on a laser beam scanning line 98. A signal (referred to as a "BD signal" hereinafter), which is output from the light sensing element 97 when the laser beam L passes by the light sensing element 97, is used to synchronize the start of an image signal in each scanning. In other words, when a predetermined time is delayed after the BD signal is given in each scanning, an unillustrated control unit starts to modulate the laser source 91 in accordance with a laser drive signal LD corresponding to the image signal so as to perform a scanning operation.
As a result, even if the scanning speed of the laser beam is changed by unevenness of the rotation speed of the constant speed rotary motor 94 or the like, the image start position on the photoconductive drum 96 in each scanning can be controlled to be substantially constant.
However, such a conventional apparatus cannot compensate for a change in the scanning speed while an image is being formed. In other words, even if, according to the conventional apparatus, the image start position in each scanning can be synchronized by modulating the light source in accordance with the image signal so as to form an image when a predetermined time is delayed after the BD signal is given, when the scanning speed is changed while scanning, an image end position changes on each scanning line and this causes what is called a jitter error.
In particular, if a large jitter error appears in a comparatively short cycle of several scanning lines, a printing position is shifted on the scanning end side, a character pattern is disarranged, and predetermined halftone density can not be obtained in a halftone output by modulating a dither, a density pattern or the pulse width, so that printing quality is considerably lowered.
The prime cause of the jitter of a short cycle is precision in the shape of each plane of the rotary polygon mirror 93. Generally, in a scanning plane which is a luminous plane formed with time by a laser beam deflected by a deflecting reflection plane, when the deflecting reflection plane is convex, the scanning speed is high, and, on the contrary, when the deflecting reflection plane is concave, the scanning speed is low. The shape of the plane is distorted by the centrifugal force in the operation of the polygon mirror 93, the rise in temperature, and the process precision determined in processing. In order to make the jitter of a short cycle sufficiently small, it is necessary to enhance the process precision, improve the method of retaining the polygon mirror and take measures against a rise in temperature and therefore design and production costs are increased.